In a cellular mobile radio system, it is necessary to keep transmitter power to the lowest level possible without impairing signal quality, so as to reduce the risk of interference between neighbouring cells which transmit on mutually the same channels. Transmission quality is therefore measured continuously and the transmitter power is adjusted to one of a number of possible transmitter power levels (for instance 8).
In known mobile telephony systems, for instance the system described in U.S. Pat. No. 4,523,155, which relates to a mobile telephone of the FDMA-type, a base station measures signal quality and sends a signal to the mobile telephone, where the digital signal is converted to an analog signal which represents the set-point value of the desired signal level. A detector measures the power output of the transmitter stage and generates a voltage which represents said power output (the actual or real value). This voltage is compared with the set-point value and a transmitter stage control voltage is generated.
Because of the wide dynamic range of the power output, the detector voltage will vary greatly and at the lowermost transmitter powers will be of the same order of magnitude as the offset voltage normally found in the differential amplifier used in the control circuit as a means for comparing the set-point value with the actual value. Consequently, the control circuit may have difficulty in distinguishing between offset voltage and the detector output signal at very low levels.